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Fast Switching (ip route-cache) during a route change.

Hi All,

The basic definition of fast switching says that whenever there is a new packet on the ingress interface  the first packet is always process switched and

outbound interface and a MAC header for this destination is tsored in a fast cache. The next time and every subsequent time a simlar packet arrives the interface

consults the Fast Cache directly to determine whether an outbound interface and a MAC header have been cached for this destination and if there is, the CPU is not interrupted and the data in the fast cache table is used to forward the packet.

I assume that the fast cache table is not updated dynamically when there is a route change like in case of CEF and is only updated when the first packet is process switched.

1) If any time after the entry is stored in the fast cache if there is route changes to a different interface what will happen ?

2) The fast cache contains the old entry so will the subsequest packets be dropped becasuse that route is no more valid?

3) How will the interface on which the packet arrives realize that the entry for this destination is now stale and the packet needs to be process switched again to revalidate the fast cache data?

Thanks in Advance !!!


Umesh Shetty

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